Abstract

Background

Previous research has documented heterogeneity in the effects of maternal education
on adverse birth outcomes by nativity and Hispanic subgroup in the United States.
In this article, we considered the risk of preterm birth (PTB) using 9 years of vital
statistics birth data from New York City. We employed finer categorizations of exposure
than used previously and estimated the risk dose-response across the range of education
by nativity and ethnicity.

Methods

Using Bayesian random effects logistic regression models with restricted quadratic
spline terms for years of completed maternal education, we calculated and plotted
the estimated posterior probabilities of PTB (gestational age < 37 weeks) for each
year of education by ethnic and nativity subgroups adjusted for only maternal age,
as well as with more extensive covariate adjustments. We then estimated the posterior
risk difference between native and foreign born mothers by ethnicity over the continuous
range of education exposures.

Results

The risk of PTB varied substantially by education, nativity and ethnicity. Native
born groups showed higher absolute risk of PTB and declining risk associated with
higher levels of education beyond about 10 years, as did foreign-born Puerto Ricans.
For most other foreign born groups, however, risk of PTB was flatter across the education
range. For Mexicans, Central Americans, Dominicans, South Americans and "Others",
the protective effect of foreign birth diminished progressively across the educational
range. Only for Puerto Ricans was there no nativity advantage for the foreign born,
although small numbers of foreign born Cubans limited precision of estimates for that
group.

Conclusions

Using flexible Bayesian regression models with random effects allowed us to estimate
absolute risks without strong modeling assumptions. Risk comparisons for any sub-groups
at any exposure level were simple to calculate. Shrinkage of posterior estimates through
the use of random effects allowed for finer categorization of exposures without restricting
joint effects to follow a fixed parametric scale. Although foreign born Hispanic women
with the least education appeared to generally have low risk, this seems likely to
be a marker for unmeasured environmental and behavioral factors, rather than a causally
protective effect of low education itself.